Turkish pages are Turkish to the end

The errors other packages give a page (store, catalog, mirror) are
shown in its language; a percentage is written as the language writes
it (%93 in Turkish); the footer's time in its date form; and a stale
form is refused with a sentence rather than "bad request".

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

internal/web/admin_nodeengine.go

@@ -21,15 +21,16 @@ func (p *AccountPages) RegisterNodeEngines(mux *http.ServeMux, a NodeEngineAdmin
 	mux.HandleFunc("POST /admin/nodes/{id}/engines", p.adminOnly(func(w http.ResponseWriter, r *http.Request, admin store.User) {
 		sess, _, _ := p.session(r)
 		if !p.accounts.CheckCSRF(sess, r.FormValue("csrf")) {
-			http.Error(w, "bad request", http.StatusBadRequest)
+			badForm(w, r)
 			return
 		}
 		node, engine, build := r.PathValue("id"), r.FormValue("engine"), r.FormValue("build")
 		q := url.Values{}
 		if err := a.SetNodeEngine(r.Context(), node, engine, build, admin.Email); err != nil {
 			p.log.Warn("an engine build was not chosen", "admin", admin.Email, "node", node, "err", err)
-			// The control plane's reason, as it gave it: in English.
-			q.Set("problem", node+": "+err.Error())
+			// The control plane's reason, in the page's language where it
+			// is one the pages know, else as it gave it.
+			q.Set("problem", node+": "+langOf(r).Err(err))
 		} else {
 			p.log.Info("an engine build was chosen", "admin", admin.Email, "node", node, "engine", engine, "build", build)
 			// Said in the language of the person who chose, since the